Miscarriage... signs, symptoms?

So, my doctor is a little concerned about my constant stomach ache and says it can be a sign of miscarriage. Though, I haven't bled or had any other bad symptoms. Just nausea and fatigue and of course occasional pains in my lower abdomen on the sides that only last some seconds and their gone. So, my question is for any mom's or women who have experienced a miscarriage.... what does it feel like? What were your warning signs and did you have a constant achy stomach? Thanks for any help! Xoxo